well first of all, add your music to library, then drag and drop the music onto the work area. Now bring up the menu at the bottom, on the right hand side of the menu you got "sound", "effect" and "sync". On "sync", set it to "stream". Also, remember to have the one layer for your music and make sure you can see the music all the way through your movie til you want it to finish (when you want it to finish, use a keyframe). It'll make sense when you look at the interface.